The invention relates to improvements in electrostatus stylus recording apparatus, and more particularly, to apparatus for reducing wear of a reusable dielectric recording element.
In U.S. Pat. No. 3,045,644, issued to F. A. Schwertz, there is disclosed an electrostatic stylus recorder which includes a reusable dielectric recording element in the form of a rotatably driven, dielectric-coated, metallic drum. Electrostatic images are formed on the drum by the collective action of a plurality of closely spaced, electrically conductive styli which are arranged to form a linear array spanning the width of the drum. The ends of the styli are positioned in contact with the dielectric surface of the recording element. When a high voltage electrical signal, representative of video information, is applied to each stylus, an electrical discharge occurs between the stylus tip and the metallic drum. During this discharge, electrical charges are deposited on the interposed surface of the dielectric coating. By controlling the application of video signals to the individual styli, an electrostatic image represenative of the video information being recorded is formed on the dielectric surface of the drum. This charge image is developed by conventional eletrographic techniques, and the resulting visible image is transferred to a paper support. The transfer step allows the dielectric drum to be repetitively cycled through the recording process to form a multitude of transferrable images.
In U.S. Pat. No. 3,301,947 issued to J. J. Stone, it is disclosed that the video signal levels required to effect the electrical discharges which characterize electrographic stylus recording can be significantly reduced by creating an RF field between the stylus tips and the conductive backing of the recording element. This RF field is of an amplitude sufficient to ionize the air in the vicinity of the stylus tip. The ionized air provides a conductive path through which current controlled by the video signal source can travel from the stylus to the dielectric surface. By using an RF field with a zero-volt symmetry, no net charge is transferred to the dielectric surface because that charge which is applied during each half-cycle of the RF field is neutralized by the charge of opposite polarity applied during the other half-cycle. Thus, only when video signals are applied to the styli does a net charge deposition result. In contrast to the aforementioned Schwertz recorder, Stone's apparatus does not feature a reusable recording element; rather, Stone forms and develops electrostatic images directly on dielectric-coated paper.
In applying Stone's teaching to electrographic stylus recorders of the type which employ reusable recording elements, one finds that an unbiased (i.e. zero symmetry) RF field has a deleterious effect on the useful life of the recording element. More specifically, one finds that undesirable grooves are gradually worn in the dielectric surface opposite each stylus tip and that, after a relatively short period of usage, e.g., ten thousand cycles of operation, the grooves are of such depth that the recording element can no longer be used to produce high quality images. I have found that these wear grooves are the result of periodic ion-bombardment which occurs during each half-cycle of the RF field. During each cycle of the RF field, relatively massive, positively charged, gas ions are accelerated by the electric field and alternately bombard the stylus tip and the dielectric surface. Such ion bombardment acts to etch away the dielectric surface and, to a lesser degree, the stylus tip.